1. Field of the Invention
This invention relates to heavy duty pneumatic radial tires exhibiting less railway wear, and is to diminish extraordinarily irregular wear, particularly railway wear, which are apt to occur in such tires when continuously travelling at a high speed over a long distance, by an improvement of a tread in the tire.
2. Description of the Prior Art
In general, radial tires provided with at least two metal cord layers as a belt reinforcement have excellent resistance to wear, puncture and the like as compared with conventional bias tires because a highly stiff belt layer is arranged between the tread rubber and the carcass ply. On the other hand, radial tires are somewhat defective in the comfort degree owing to the rigid reinforcing effect with such belt. Accordingly, these radial tires have usually been developed for use on good roads as distinguished from unimproved ones. Recently, the demand for such tires has considerably increased in association with the remarkable improvement of road circumstances such as the development of networks of highways and the like.
In such applications, zigzag-type ribs extending circumferentially of tire are usually provided in the tread of tire. Generally, tires having such a tread pattern are called as a rib-type tire.
In the rib-type tire, ribs are usually continuous toward the circumferential direction of tire and may be discontinuous toward the circumferential direction due to the presence of traverse grooves arranged along the widthwise direction of tire. In any case, when a vehicle provided with such tires goes continuously straight on a highway at a high speed over a long distance, there are caused extraordinarily irregular wear (hereinafter referred to as eccentric wear) which have never been observed under the conventional common travelling conditions.
Namely, as shown in FIG. 1, the eccentric wear is locally caused in a shadowed region A near a top of a convex part 3 of a circumferential rib 2 formed in a tread of a tire T, said convex part being projected in a widthwise direction of the tire T toward a groove 1 extending zigzag along a circumferential direction of the tread, and then gradually increases to form a region A having stepwise height h and a width w in section as shown in FIG. 2. The region A of the eccentric wear gradually grows with the increase of the travelling distance and finally communicates with adjoining regions A. As a result, these regions are continuously joined with each other in the circumferential direction of tire T. Moreover, the stepwise height h and the width w are gradually enlarged with the increase of the travelling distance.
The above eccentric wear is generally called railway wear, which produces not only the recess of the groove 1 to render the appearance of the tire T awkward, but also considerably deteriorates the life of tire.
The eccentric wear begins to occur only in the vicinity of the top at the convex part 3 of the zigzag-type circumferential rib 2 and does not start from a concave part 4 of the circumferential rib 2 in opposition to the convex part 3 along the widthwise direction of the tire T. However, the concave part 4 is also subjected to railway wear in due time with the evolution of the eccentric wear.